The invention relates to a sector-shaped screen segment for rotating disk filters or thickeners with a folded screen for thickening fiber suspensions.
In the production of wood pulp and cellulose and the processing of waste paper and other fibrous materials, a fiber suspension is often treated at substance densities of between approximately 0.3% and approximately 3%. The further processing of the raw materials by grinding or bleaching and the temporary storing of the fibrous material in vats require a higher concentration which is economically obtainable with rotating disk filters or thickeners with a folded screen. In both cases, a drum which rotates about a horizontal axis dips into a trough into which the fiber suspension which is to be thickened is continuously introduced. The drum which is covered on the outside with a woven screen is of accordion-like configuration to increase the screen area. In a known disk filter, such as, for example, that offered by Hedemora Pulp Machinery, the drum is comprised of several disk-shaped units which are mounted on a common, rotatably mounted and driven supporting pipe and each consists of several sector-shaped screen segments which are contiguous along radii and are covered on the outer side with the woven screen which together with screen carriers defining a cavity, forms the end faces of the filter disks.
As the disk filter dips into the fiber suspension and turns slowly in it, water passes out of the fiber suspension through the screen into the interior of the rotating drum, and a fibrous fleece forms on the outer side of the screen and increases the filtering effect. The arrangement is designed to produce between the level of the fiber suspension in the trough and the level of the water flowing off from the drum interior through the supporting pipe, a hydrostatic pressure difference which causes continuous filtration. To remove from the filter disks the continuously increasing layer of fibers accumulating on the screen as the fiber suspension in the trough passes through it, there are arranged on the trough, above the fiber suspension to be filtered, rake-like scrapers which form a kind of chute on which the layers of fibers which have been removed from the filter disks slide to a location outside the trough. Removal of the layers of fibers from the screen may be facilitated by provision of a spraying pipe above the disk filter to spray off and thereby remove the layers of fibers from the screen.
In the known disk filter described above, the screen segments have a kind of frame with two lateral, radially extending legs and a circumferentially outer leg, and the screen carriers arranged in axially spaced relation to each other consist of a welded wire grid with a woven plastic screen resting on it. The screen forms a filter bag which has a zip fastener and has been shrunk onto the frame and the screen carriers.
In known thickeners with a folded screen, the circumferentially extending folds are less deep than in a disk filter. In another known disk filter, the screen carriers consist of perforated metal plates with apertures of relatively large diameter.
The known sector-shaped screen segments are disadvantageous in several respects: Firstly, the supporting structures for the woven screen consisting of frame and screen carriers involve quite high manufacturing expenditure, which results in relatively high costs. In addition, these screen structures are not particularly stable --it must be remembered that the filter disks have a diameter of the order of magnitude of 2 m --and even if the rotating filter disks run slightly out of true at the sides, this causes the woven screen to run sideways against the scrapers removing the layer of fibers from the drum, the spraying pipes and the like, and, for this reason, in the known disk filter described above, the filter disks are guided at the sides in the area in which the layer of fibers is removed. Finally, relative motions between the woven screen and the screen carriers supporting it result in wear and tear of the screen and hence in decreased service life.
The object underlying the invention is to create a sector-shaped screen segment (viewed from the side, the known screen segments described above have approximately the shape of the sector of a circle with the top cut off) for rotating disk filters or thickeners with a folded screen for thickening fiber suspensions which is simpler and hence less expensive to manufacture, particularly also as far as attachment of the screen is concerned.